fix: golangci-lint v2 config and lint-clean production code

- Fix .golangci.yml for v2 format (linters-settings -> linters.settings)
- All production code now passes golangci-lint with zero issues
- Line length 88, funlen 80/50, cyclop 15, dupl 100
- Extract shared helpers in db (scanChannels, scanInt64s, scanMessages)
- Split runMigrations into applyMigration/execMigration
- Fix fanOut return signature (remove unused int64)
- Add fanOutSilent helper to avoid dogsled
- Rewrite CLI code for lint compliance (nlreturn, wsl_v5, noctx, etc)
- Rename CLI api package to chatapi to avoid revive var-naming
- Fix all noinlineerr, mnd, perfsprint, funcorder issues
- Fix db tests: extract helpers, add t.Parallel, proper error checks
- Broker tests already clean
- Handler integration tests still have lint issues (next commit)

Makefile

@@ -1,7 +1,7 @@
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
 LDFLAGS := -ldflags "-X main.Version=$(VERSION)"
 
-.PHONY: build test clean docker
+.PHONY: build test clean docker lint
 
 build:
 	go build $(LDFLAGS) -o chatd ./cmd/chatd/
@@ -13,5 +13,8 @@ test:
 clean:
 	rm -f chatd chat-cli
 
+lint:
+	GOFLAGS=-buildvcs=false golangci-lint run ./...
+
 docker:
 	docker build -t chat:$(VERSION) .